Biomedical Engineering Reference
In-Depth Information
surface microporosity, while bone was not observed on the surface
of dense bioceramics [616]. Furthermore, implantation of porous
β-TCP bioceramics appeared to induce bone formation in soft tissues
of dogs, while no bone formation was detected in any α-TCP implants
[613]. More to the point, titanium implants coated by a microporous
layer of OCP were found to induce ectopic bone formation in goat
muscles, while a smooth layer of carbonated apatite on the same
implants was not able to induce bone formation there [619, 620].
Although the underlying mechanism(s) leading to bone induction
by synthetic materials remains largely unknown, the dissolution/
precipitation behavior of calcium orthophosphates [532], as well as
their microporosity [621-623], chemistry [621], the specific surface
area [623], as well as surface topography and geometry have been
pointed out as the relevant parameters. A positive effect of increased
microporosity on the ectopic bone formation could be direct and
indirect. Firstly, an increased microporosity is directly related to the
changes in surface topography, i.e. increases a surface roughness,
which might affect the cellular differentiation. Secondly, an increased
microporosity indirectly means a larger surface that is exposed to the
body fluids leading to elevated dissolution/precipitation phenomena
as compared to non-microporous surfaces. Furthermore, other
hypotheses are available. Namely, Reddi explained the apparent
osteoinductive properties as an ability of particular bioceramics to
concentrate bone growth factors, which are circulating in biological
fluids, and those growth factors induce bone formation [624].
Other researchers proposed a similar hypothesis that the intrinsic
osteoinduction by calcium orthophosphate bioceramics is a result
of adsorption of osteoinductive substances on their surface [618].
Moreover, Ripamonti [625] and Kuboki et al. [626] independently
postulated that the geometry of calcium orthophosphate bioceramics
is a critical parameter in bone induction. Specifically, bone induction
by calcium orthophosphates was never observed on flat bioceramic
surfaces. All osteoinductive cases were observed on either porous
structures or structures contained well-defined concavities. What's
more, bone formation was never observed on the peripheries of
porous implants and was always found inside the pores or concavities,
aligning the surface [194]. Some researchers speculated that a low
oxygen tension in the central region of implants might provoke
a dedifferentiation of pericytes from blood micro-vessels into
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